Spherically mounted retroreflector having an embedded temperature sensor and socket
A spherically mounted retroreflector (SMR) having an embedded temperature sensor and a socket for attaching to an electrical connector.
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The present invention relates in general to methods for measuring spherically mounted retroreflectors (SMRs) and in particular to methods for determining surface coordinates and three-dimensional distances based on measurements of SMRs.
BACKGROUNDThere is a class of instruments that measures the coordinates of a point by sending a laser beam to a retroreflector target in contact with the point. The instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an ADM or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. There is another category of instruments known as total stations or tachymeters that may measure a retroreflector or a point on a diffusely scattering surface. Laser trackers, which typically have accuracies on the order of a thousand of an inch and as good as one or two micrometers under certain circumstances, are usually much more accurate than total stations. The broad definition of laser tracker, which includes total stations, is used throughout this application.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the SMR. In most cases, the term SMR is applied to a cube-corner retroreflector embedded within a metal sphere. However, the term SMR may also be applied to a cateye retroreflector embedded with a metal exterior spherical portion. Such a cateye retroreflector may be constructed in a shape of a sphere or in the shape of two adjoining hemispheres. A cube-corner retroreflector includes three mutually perpendicular reflectors. The vertex, which is the common point of intersection of the three reflectors, is located near the center of the sphere. In its normal tracking mode, the laser tracker sends a beam of light from the tracker to a position near the vertex of the SMR. As long as the beam of light strikes the vertex, the beam of returning beam of light retraces the path of the outgoing beam of light back to the tracker. If the beam of light strikes the SMR slightly off the vertex, the beam of light will return parallel to, but not exactly coincident with, the outgoing beam of light. A servo system within the tracker adjusts the direction of the beam emitted by the tracker to bring it back to the center, thereby allowing the beam to follow a moving retroreflector. Because the vertex is nearly coincident with the sphere center of the SMR, the perpendicular distance from the vertex to any surface on which the SMR rests remains nearly constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface to a relatively high accuracy by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to characterize the 3D coordinates of a surface.
An SMR may also be used to measure the distance between two nests. A particularly useful kind of nest is a kinematic nest, which has the property that an SMR can be repeatably positioned in the nest. One type of nest makes contact with the SMR surface at three points. Some types of nests are magnetic nests that hold the SMR securely in place against the nest contact points.
Some laser trackers have the ability to measure six degrees of freedom (DOF), which may include three translations, such as x, y, and z, and three rotations, such as pitch, roll, and yaw. An exemplary six-DOF laser tracker system is described in U.S. Pat. No. 7,800,758 ('758) to Bridges, et al., incorporated by reference herein. The '758 patent discloses a probe that holds a cube corner retroreflector, onto which marks have been placed. The cube corner retroreflector is illuminated by a laser beam from the laser tracker, and the marks on the cube corner retroreflector are captured by an orientation camera within the laser tracker. The three orientational degrees of freedom, for example, the pitch, roll, and yaw angles, are calculated based on the image obtained by the orientation camera. The laser tracker measures a distance and two angles to the vertex of the cube-corner retroreflector. When the distance and two angles, which give three translational degrees of freedom of the vertex, are combined with the three orientational degrees of freedom obtained from the orientation camera image, the position of a probe tip, arranged at a prescribed position relative to the vertex of the cube corner retroreflector, can be found. Such a probe tip may be used, for example, to measure the coordinates of a “hidden” feature that is out of the line of sight of the laser beam from the laser tracker.
As explained hereinabove, the vertex of a cube corner retroreflector within an SMR is ideally placed at the exact center of the sphere into which the cube corner is embedded. In practice, the position of the vertex is off the center of the sphere by up to a few thousandths of an inch. In some cases, the difference in the positions of the vertex and the sphere center are known to high accuracy, but this data is not used to correct the tracker readings. In the accurate measurements made with laser trackers, this error in the centering of the cube corner retroreflector in the sphere is sometimes larger than the errors from the distance and angle meters within the laser tracker. Consequently, there is a need for a method to correct this centering error.
Most of the SMRs in use today contain open-air cube corner retroreflectors. There are some SMRs that use glass cube corner retroreflectors, but in most cases these have limited accuracy. Because of the bending of the light entering such glass cube corners, the light appears to travel in a direction that is not the true direction within the cube corner. Consequently, SMRs made with glass cube corners tend to be made very small, as this reduces error, and they tend to be used in applications where the highest accuracy is not required. A method for minimizing this error using a six-DOF laser tracker is given in U.S. Pat. No. 8,467,072, the contents of which are incorporated by reference.
In many cases, the SMR of interest is an open-air cube corner rather than a glass cube corner, and the laser tracker measures only three degrees of freedom rather than six. Measurement error associated with such an SMR and tracker combination result both from errors in vertex centering and in sphere diameter. These errors can be corrected to an extent by purchasing a more expensive SMR having smaller centering and radius errors, but the errors cannot be eliminated. Furthermore some of the errors arise from temperature changes in the SMR. There is a need for a method to correct these errors, including errors resulting from SMR temperature changes.
SUMMARYAccording to one aspect of the invention, a probing system comprises a spherically mounted retroreflector (SMR), the SMR including a body, a retroreflector, and a temperature sensor, the body having a spherical exterior portion that has a sphere center, the body containing a cavity, the cavity sized to hold the retroreflector, the cavity open to a region outside the body, the retroreflector at least partially disposed in the cavity, the retroreflector having a set of three mutually perpendicular planar reflectors that intersect in a first set of three lines and in a common vertex point, there being a first distance between the vertex point and the sphere center, the temperature sensor affixed to the SMR and in thermal contact with the SMR, the SMR further including a socket rigidly affixed to the body, the socket in electrical contact with the temperature sensor, the socket configured to accept a first electrical connector, the socket accessible to the first electrical connector from an outside surface of the SMR, the first electrical connector attached to a first end of an electrical cable.
Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several figures, in which:
An exemplary laser tracker 10 is illustrated in
The device frame of reference 30 of the laser tracker 10 is fixed with respect to the azimuth base 16, which is typically stationary with respect to the tracker's surroundings. The device frame of reference 30 may be represented in a variety of coordinate systems. It may be represented in a Cartesian coordinate system having three perpendicular axes x′, y′, and z′. It may be represented in a spherical coordinate system, a point 74 being represented by a radial distance 73 (r), a first (zenith) angle 72 (θ), and a second (azimuth) angle 71 (φ). The angle θ is obtained by using the projection of the point 74 onto the z axis. The angle φ is obtained by using the projection of the point 74 onto the x′-y′ plane. The laser tracker 10 inherently measures in a spherical coordinate system using one distance meter to measure r and two angular encoders to measure θ and φ. However, a point measured in spherical coordinates may be easily converted to Cartesian coordinates. In an embodiment, the gimbal point 22 is selected as the origin. Other coordinate systems are possible and may be used.
The laser tracker 10 also includes rotating frames of reference. One of the rotating frames of reference is the payload frame of reference 35 that aims the beam of light 46 toward the SMR 26. The payload frame of reference rotates about the axis 20 and the axis 18. It should be understood that the term payload frame of reference may refer to the final beam delivery portion of any type of beam delivery system, not just the payload 15 of
The SMR 26 has an SMR frame of reference 40. The SMR frame of reference may be represented, for example, in a Cartesian coordinate system having three perpendicular axes x, y, and z. The x, y, and z axes of the SMR frame of reference 40 move with the SMR 26 and are not in general parallel to the corresponding axes x″, y″, and z″ of the payload frame of reference 35. In an embodiment, a vertex of a cube corner retroreflector within the SMR is the origin of the SMR frame of reference. The SMR 26 may be placed in contact with the workpiece surface 61 at a point 63. To find the three-dimensional (3D) coordinates of the point 63, the tracker first determines the 3D coordinates of the vertex of the SMR 26 using the distance and two angles it has measured. It then shifts the 3D coordinates of the SMR vertex toward the surface by an amount equal to the sphere radius of the SMR. The SMR 26 may also be used to measure 3D coordinates when placed on a kinematic nest, as further explained hereinbelow.
Laser beam 46 may include one or more laser wavelengths, or the beam 46 may be a beam of light other than laser light. For the sake of clarity and simplicity, a steering mechanism of the sort shown in
In exemplary laser tracker 10, locator cameras 52, 56 and light sources 54 are located on payload 15. Light sources 54 illuminate one or more retroreflector targets 26. In an embodiment, light sources 54 are LEDs electrically driven to repetitively emit pulsed light. Each locator camera 52 and 56 includes a photosensitive array and a lens placed in front of the photosensitive array. The photosensitive array may be a CMOS or CCD array, for example. In an embodiment, the lens of locator camera 52 has a relatively wide field of view, for example, 30 or 40 degrees. In contrast, the lens of locator camera 56 may have a relatively narrow field of view, for example, to enable clear reading of a bar code or a serial number of an SMR 26 held in a home nest 17, as discussed further hereinbelow. The purpose of the lens in a camera 52, 56 is to form an image on the photosensitive array of objects within the field of view of the lens. The image on the photosensitive array is sent to electronics, which may be internal or external to the photosensitive array, to provide an electrical signal to a processor. The electrical signal is evaluated by the processor to extract relevant information such as images, text, array locations, and so forth. Usually at least one light source 54 is placed near a locator camera 52 so that light from light source 54 is reflected off each retroreflector target 26 onto the locator camera 52. In this way, retroreflector images are readily distinguished from the background on the photosensitive array as their image spots are brighter than background objects and are pulsed. In an embodiment, there are two locator cameras 52 and two light sources 54 placed about the line of laser beam 46. By using two locator cameras 52 in this way, the principle of triangulation can be used to find the three dimensional coordinates of any SMR within the field of view of the locator camera. In addition, the three dimensional coordinates of an SMR can be monitored as the SMR is moved from point to point.
For a locator camera 56 designed to read bar codes, a light source is not placed close to the camera as the bright flash of retroreflected light would prevent the camera from reading the much dimmer lines of a bar code. Instead, the lights 54, which are too far away from the locator camera 56 to retro-reflect the light from the retroreflector target 26 into the locator camera 56, may be used to illuminate the bar codes, if needed.
As shown in
The laser tracker 10 measures a distance r using either an interferometer or an ADM. It measures an azimuth angle φ and a zenith angle θ using angular encoders. Hence the laser tracker measures in a spherical coordinate system, although the coordinate values for any measured point may be converted into coordinates in any other desired coordinate system, for example, the Cartesian coordinate system 30 of
It should be similarly understood that three degrees of translational freedom means three independent degrees of translational freedom. Another way of saying this is that the three directions corresponding to the three degrees of translational freedom form a basis set in three-dimensional space. In other words, each of the three directions corresponding to a degree of translational freedom has a component orthogonal to each of the other two directions.
Light from source element 410 passes through beam splitter 420. Light from source element 405 reflects off mirror 415 and beam splitter 420. If source elements 405, 410 contain light of different wavelengths, beam splitter 420 may advantageously be a dichroic beam splitter that transmits the wavelength of light emitted by source element 410 and reflects the wavelength of light emitted by source element 405.
Most of the light from beam splitter 420 passes through beam splitter 425. A small amount of light is reflected off beam splitter 425 and is lost. The light passes through beam expander 435, which expands the size of the beam on the way out of the tracker. Expanding the beam of light is helpful because it enables the light to propagate a longer distance with less change in beam size. The laser light 440 leaving the tracker 10 travels to a retroreflector target 26. A portion of this laser light reflects off the retroreflector 26 and returns to the tracker. The beam expander 435 reduces the size of the beam on the way back into the tracker.
Part of the returning light travels to beam splitter 425. Most of the light passes on to elements 405, 410 but a small amount is split off and strikes position detector 450. In some cases, the light may pass through a lens after reflecting off beam splitter 425 but before striking position detector 450. The position detector 450 may be of several types—for example, a position sensitive detector or photosensitive array. A position sensitive detector might be a lateral effect detector or a quadrant detector, for example. A photosensitive array might be a CMOS or CCD array, for example. Position detectors are responsive to the position of the returning light beam. The motors attached to the azimuth mechanical axes and the zenith mechanical axes are adjusted by a control system within the tracker 10 to keep the returning light beam centered, as nearly as possible, on the position detector 450.
The SMR 26 includes a body having a spherical exterior portion and a retroreflector. The spherical exterior portion contains a cavity sized to hold a cube corner retroreflector, which is at least partially disposed in the cavity. The spherical exterior portion has a spherical center. A cube corner retroreflector may be an open-air cube corner or a glass cube corner. An open-air cube corner retroreflector has an interior portion of air, while a glass cube corner retroreflector has an interior portion of glass.
A cube corner retroreflector includes three planar reflectors that are mutually perpendicular. The three planar reflectors intersect at a common vertex, which in the ideal case is a point. Each of the planar reflectors has two intersection junctions, each intersection junction of which is shared with an adjacent planar reflector for a total of three intersection junctions within the cube corner retroreflector. The cube corner retroreflector has an interior portion that is a region of space surrounded on three sides by the planar reflectors. In the case of an open-air cube corner retroreflector, which is the subject of the present application, the cavity includes an air-filled portion interior to the three planar reflectors, the three intersection junctions, and the vertex. The cavity is open to an exterior of the body, which provides a means for light to be sent into and reflected from the retroreflector.
There are at least three common methods for making open-air cube corner retroreflectors: a replication process, a mirror insertion process, and an ECM process.
The second common method of making open-air cube corner retroreflectors is the mirror insertion process in which mirror panels joined into a cube-corner assembly are inserted into a cavity in the spherical exterior portion. Three mirror panels are joined together to be mutually perpendicular.
The third common method of making open-air cube corner retroreflectors is the electrochemical machining (ECM) process. In some cases, the retroreflector and the spherical exterior portion are integrated into a single unit. Such an SMR may be created, for example, by removing the cavity using a combination of traditional machining and ECM. Such an ECM process may be used to create three mutually perpendicular surfaces that are flat and smooth. Such surfaces may be covered with a reflective coating such as gold or silver to provide the three reflective surfaces.
An SMR having an open-air cube corner retroreflector is illustrated in
An axis of symmetry 840 is symmetrical with respect to the three intersection lines 810A, 810B, and 810C. The angle between the axis of symmetry and any of the three intersection lines is cos−1(1√{square root over (3)})=54.7°. A runout plane 865 that passes through the sphere center 860 is drawn perpendicular to the axis of symmetry 840. The axis of symmetry 840 intersects the runout plane 865 in an intersection point 870. An SMR error vector 885, which extends from the vertex 820 to the sphere center 860, decomposes into two vector components: an SMR depth error vector component 880 that extends from vertex 820 to the intersection point 870 and an SMR runout error vector component 890 that extends from the intersection point 870 to the sphere center 860. The SMR depth error vector component 880 has a length equal to an SMR depth error, and the SMR runout error vector component 890 has a length equal to an SMR runout error. The SMR depth error vector component 880 and the SMR runout error vector component 890 are shown in
The elements of
A magnified perspective view near the center of the SMR is shown in
A beam reference ray 972 is a ray coincident with the line of intersection between the beam reference plane 975 and the beam runout plane 965, wherein the beam reference ray 972 begins at the beam intersection point 970 and is directed along the half of the beam reference plane 975 that includes the reference point 932. The angle between the beam reference ray 972 and the beam runout error vector 976 is the beam runout reference angle 982. To the extent that the beam of light 46 is not aligned with the axis of symmetry 840, there will be a difference between the calculated 3D coordinates of the sphere center 978 and the actual 3D coordinates 860 of the sphere center. This difference is discussed further with reference to
Measurements are performed on each SMR to determine the position of the sphere center relative to the vertex. The results of such measurements may be described in several different ways. One such description of the sphere center relative to the vertex point includes the SMR depth error 880, the SMR runout error 890, and the SMR runout reference angle 950. A different but equivalent description includes component lengths of the SMR error vector using Cartesian coordinates. For example, such component lengths may be given along Cartesian axes x, y, z within a frame of reference of the SMR such as the frame of reference 730 of
A Cartesian frame of reference may also be applied to the beam runout plane 965.
There are many ways to find the position of the sphere center 860 relative to the vertex 820. One method is to measure the SMR with a Cartesian coordinate measuring machine (CMM). With this method, the position of the vertex relative to the sphere center can be found to a fraction of a micrometer. For example, with a very good Cartesian CMM, an expanded uncertainty of the position of the center relative to the vertex may be better than 0.4 micrometer along each of three Cartesian axes x, y, z. This error is much less than the centering error of an SMR, which, in a typical SMR is 0.0005 inch=12.7 micrometers along the axes x, y, z. In a very good SMR, the specified centering error components may be as small as 0.0001 inch=2.54 micrometers.
Another way to measure the depth of the sphere center 860 relative to the vertex 820 makes use of an absolute interferometer or other type of ADM having a high accuracy. In an embodiment, a kinematic nest configured to repeatably center a spherically shaped object is arranged to push upwards on an SMR. A reference SMR is measured with an accurate Cartesian CMM to find the SMR error vector 885. The reference SMR is placed in the nest and the absolute interferometer is used to measure the distance along a horizontal line from the absolute interferometer to the vertex of the SMR. An SMR under test is next placed in the nest and the measurement repeated. Let the SMR depth error of the reference SMR, EDepthRefSMR, be taken as generally positive in the direction from a distance measuring device to the SMR. Then, following the above procedure, the SMR depth error of a test SMR is
EDepthTestSMR=dTestSMR−dRefSMR+EDepthRefSMR, (1)
where dTestSMR, dRefSMR are the measured distances to the SMR under test and the reference SMR, respectively.
Another way to measure the SMR runout error and SMR runout reference angle (or, equivalently, the Cartesian errors in the SMR runout plane) is to rotate an SMR under a microscope according to a method well known in the art. A description of this method is given in Section B-2.1 of Appendix B of ASME Standard B89.4.19-2006, Performance Evaluation of Laser-Based Spherical Coordinate Measurement Systems, which is incorporated by reference herein. With this method, an SMR under test is placed on a kinematic nest that rests on a microscope stand. A light source illuminates the frame of the microscope. The focus is adjusted to view a speck of dust (or other small object) on the microscope frame. An operator rotates the SMR about the sphere center within the kinematic nest and observes on the microscope the radius of the runout circle. The observed radius is divided by four to get the SMR runout error, which is the magnitude of the SMR runout error vector 890. The procedure discussed in this paragraph is used as a method of determining whether an SMR meets it runout (centering) specifications. For example, a manufacturer may provide a specification for an SMR stating that the SMR has a centering error of less than 0.0005 inch. This would be interpreted to mean that the SMR has an SMR depth error of less than 0.0005 inch and SMR runout error of less than 0.0005 inch. Although the depth error and the runout error have been measured in the past for SMRs, provision has not heretofore been made to use these values by a processor to correct readings of device 10 based on SMR compensation parameters, the compensation parameters which might include SMR depth error and SRM runout error vector component information.
The observed position of the SMR runout error relative to a reference mark on the SMR may be used to determine the SMR runout reference angle 950. In an embodiment, discussed further with respect to
Besides errors in SMR centering (SMR depth error and SMR runout error vector component), there are also errors in SMR radius. In other words, the SMR radius is not exactly that indicated in a manufacturer's specifications. As an example, some high quality SMRs are manufactured from grade 25 steel balls having a diameter tolerance of ±0.0001 inch=±2.54 micrometers, which is equivalent to a radius tolerance of ±1.27 micrometers. In other words, for an SMR manufactured with this type of steel ball, the actual SMR radius is expected to lie within ±1.27 micrometers of the nominal (specified) radius, at least at portions of the spherical surface not too close to the cavity that holds the retroreflector.
There are several ways to measure the radius of an SMR. A Cartesian CMM can be used to accurately measure the radius of an SMR under test. The radius error is found by taking the difference between the measured radius and a reference or nominal radius.
An absolute interferometer may also be used to measure the radius error of an SMR under test. With this method, a reference sphere (ball) is measured with a coordinate measuring instrument such as a Cartesian CMM or Talyrond® roundness measuring device to find the radius of the reference sphere. In an embodiment, a kinematic nest configured to repeatably center a spherically shaped object is arranged to push upwards on a spherical surface. The reference sphere is placed in the nest and the absolute interferometer focused onto the sphere surface along a line normal to the surface. The absolute interferometer measures a distance from the interferometer to the surface. An SMR under test is next placed in the nest. The SMR is rotated to enable the beam from the absolute interferometer to be collinear with a normal vector to the spherical exterior portion 720, and the absolute interferometer is used to measure the distance. The difference between the measured distance to the SMR under test and the measured distance to the test sphere is the radius error.
Changes in temperature of the SMR may cause the vertex 820 to shift its position relative to the sphere center 860. Such changes in SMR temperature may result from (1) changes in the ambient temperature of the air surrounding the SMR, (2) heating of the SMR by the operator's hand, and (3) contact of the SMR with a relatively warm object such as a home position nest 17 of a laser tracker 10 of
With some types of SMRs, the effect of temperature may be relatively small. For example, as discussed hereinabove, one type of SMR is made of a single piece of steel into which the three mutually perpendicular surfaces are etched using ECM. With this type of SMR, the change in the SMR depth error with temperature will equal the coefficient of thermal expansion (CTE) times the change in SMR temperature times the initial SMR depth error. If the initial SMR depth error is 0.001 inch=25.4 micrometers and the SMR is made of steel having a coefficient of thermal expansion of 11.5 micrometers/meter/° C., the change in the SMR depth error over a 30 degree Celsius change in SMR temperature is (25.4×10−6)(11.5)(30) micrometer=0.009 micrometer, which is a negligible amount.
With some other types of SMRs, the thermal effects are larger. For example, consider the type of SMR that includes an aluminum slug put into a 1.5 inch (38.1 mm) diameter spherical exterior portion of steel. Suppose that the aluminum slug extends 10 mm below the vertex and is glued to the steel portion at that position. Neglecting the thermal expansion of the glue bond and considering a point of contact at which the axis of symmetry intersects the spherical exterior portion, the relative change in the vertex position with temperature is the difference in the CTE values for aluminum and steel times the 10 mm extension times the change in temperature. Suppose that the CTE of steel is 11.5 micrometers/meter/° C., the CTE of aluminum is 23 micrometers/meter/° C., the depth extension is 10 mm, and the change in temperature is 30° C. The change in the vertex relative to the sphere center is then (0.01)(23−11.5)(30) micrometers=4.45 micrometers.
To correct for the movement of the SMR vertex as a result of thermal expansion, a temperature sensor such as a thermistor or RTD may be embedded in the SMR. In an embodiment shown in
Use of an embedded temperature sensor 1034 with a sensor cable 1040 has the advantage of avoiding the need for a battery, temperature circuit, or wireless communications system within the SMR. With this system an SMR might be checked, for example, when the SMR is returned to a tracker home position.
As shown in
In an embodiment shown in
Numerical values used to compensate for imperfections in SMRs as a way of enabling more accurate 3D measurements are referred to as SMR compensation parameters. SMR compensation parameters are typically stored in memory of a measurement device or a computing device. For example, SMR compensation parameters may be stored in a memory of the laser tracker 10 of
In some cases, compensation calculations are made within a processor of a measurement device such as a laser tracker. For example, the tracker processor may identify the SMR being used for a particular measurement and automatically perform compensation calculations for that SMR to transform the measured vertex 3D coordinates to sphere center 3D coordinates. In other cases, compensations are made by application software. For example, application software may consider the position and orientation of the SMR in making a plurality of measurements and apply compensation calculations for the SMR radius to eliminate errors. In general, the term processor may be understood to mean a processor in a device such as a tracker, a processor in an external computer, or a processor in both.
There are several ways by which SMR compensation parameters can be entered into memory. In an embodiment, SMRs shipped with the device 10 from the factory come with compensation parameters pre-loaded into memory. In another embodiment, the user is provided with a list of numerical compensation values for each SMR. Application software embedded within the device 10 provides a means by which the user may enter the numerical values. Such values need only be entered once, as they are stored in memory within the device 10. In another embodiment, the SMR compensation parameters are provided on a flash drive, CD ROM, or other medium read by device 10, computer 80, or other component for automatic storage in memory. In another embodiment, numerical values for compensation parameters are downloaded into the tracker over a network. In another embodiment, SMR compensation parameters are encoded into a one-dimensional or two-dimensional barcode 630. A barcode reader may read the SMR compensation values on the barcode and automatically download these into memory in the device 10.
In an embodiment, a close-range camera 56 shown in
Inasmuch as the resetting of SMR distance at the home position is routine, a measurement of the barcode on the SMR can be made in conjunction with the home position measurement without requiring extra steps on the part of the operator. In an exemplary method, the SMR is placed in the home position. The tracker first turns on the camera to view the barcode and extract the SMR parameters. It then turns on the beam of light, sends it to the vertex, measures the distance, and resets the distance to a home reference value. To reset the distance meter to the home reference value in the most accurate possible way, in an embodiment, software within the tracker sets the distance value to be applied at the SMR sphere center rather than the vertex point. This is important because it ensures that the home reference value is correctly applied for all SMRs, regardless of the SMR depth error. To determine the sphere center as accurately as possible, the SMR should be placed in a preferred orientation within the home position nest 17 or the camera 56 should be used to automatically determine the orientation of the SMR in the home position nest. These methods of establishing the position of the SMR in the home position nest are discussed in more detail hereinbelow. In
In an embodiment, SMR compensation parameters are encoded into an RFID tag 650. The encoded information is retrieved using an RFID reader, which may be attached to the payload 12, perhaps in place of the close-range camera 56. The retrieved serial number may be used to access the SMR data from a networked system (the Cloud) or from a database of information saved within a device such as a tracker or computer.
In the general case, a 3D coordinate measurement device such as laser tracker 10 obtains measured 3D coordinates of the SMR vertex, for example, by measuring a distance and two angles to the SMR. The SMR compensation factors are applied to the measured 3D coordinates of the vertex to obtain the 3D coordinates of the sphere center using methods now described. In making a measurement, the operator holds the reference point 932 so as to place the SMR reference plane 920 in a preferred orientation. A preferred orientation may be selected in several different ways, depending on the measurement objectives.
In general, the operator will endeavor to align the axis of symmetry to the direction of the beam of light from the device. This was discussed hereinabove in reference to
There are some special cases in which it is not possible to align the axis of symmetry to the beam of light. For example, with the SMR at the home position 17, it may not be possible to align the axis of symmetry to the beam of light because of mechanical constraints. In this special case, the meaning of the term preferred orientation is modified accordingly to permit for the necessary change in alignment. The special case for alignment at the home position is discussed hereinbelow.
A first type of preferred orientation is one in which the SMR reference plane 920 is aligned to the x″-z″ plane of the payload frame of reference 35 as shown in
A second type of preferred orientation is one in which the SMR reference plane 920 is aligned to the y″-z″ plane of the payload frame of reference 35 as shown in
A third type of preferred orientation is one in which the SMR reference ray 940 is aligned in a prescribed manner to a measurement line to measure a dimensional characteristic of the measurement line. This is discussed further hereinbelow with reference to
We now discuss with regard to
An SMR in
The SMR 720 in
During normal operation, an operator can usually keep the axis of symmetry 840 of a 1.5-inch diameter SMR aligned to the direction of the beam of light 1240 to within about 10 degrees. As shown in
Referring now to
In a second method shown in
In a method shown in
Previously in reference to
As explained hereinabove, establishing a preferred orientation has two aspects. In the usual case, the axis of symmetry of the SMR is aligned as well as possible to the beam of light. In a second aspect, the SMR is rotated about the axis of symmetry to place the reference mark in a preferred orientation.
In some cases, it is not possible to completely align the axis of symmetry to the beam of light. For example, at a home position 17 and at some kinds of nests, mechanical constraints may prevent the SMR from being rotated into exact alignment with the beam. In the case of the home position, another type of alignment criterion may be given. For example, a manufacturer may specify that the collar of an SMR is to be placed approximately 2 mm above the nest. The software in the device 10 may then calculate the error vector based on the provided SMR parameters.
For the case in which the axis of symmetry is not aligned with the beam of light, the orientation is easily obtained by moving the reference point into the desired orientation (for example, into a vertical plane or a plane that includes the azimuth axis) and then aligning the axis of symmetry in relation to the beam direction as specified (e.g., by moving a collar 2 mm above a nest). Another possibility is to use the camera 56 to determine the orientation of the SMR. With the camera turned to point at the home position, the orientation of the SMR is easily found from the position of the barcode (or any other marker). Use of the camera in this way provides accurate results and is simple for an operator.
This camera concept may also be applied for the case of SMRs located at distances of many meters from the device 10 if suitable zoom cameras are provided in the camera. An example of such a zoom camera is described in the '758 patent, discussed above and incorporated by reference. A reflective mark such as the mark 610 of
An SMR may be used to measure the coordinates of surface points by bringing the SMR into contact with the surface at a plurality of points. It may also be used to measure the coordinates associated with kinematic nests. In both of these types of measurements, an error in the radius of the SMR can produce an error in the measurements of surface or nest points. As explained hereinabove, methods are available to accurately measure the radius of an SMR. The radius delta, defined as the measured radius minus the nominal radius, may be stored in memory for later access. The measured radius in this case is measured by one of the accurate methods described hereinabove, for example, by using a Cartesian CMM, Talyrond®, or absolute interferometer. The radius delta may also be saved in an information storage device such as a barcode or RFID chip, which in an embodiment is attached to each SMR, and read by a reading device of the 3D coordinate measurement device.
For a device 10 used to measure 3D coordinates on a surface with an SMR, the SMR is moved to several locations on the surface, and a collection of 3D coordinates for the sphere centers of the SMR obtained for the corresponding surface contact points. 3D center coordinates in close proximity to one another are used to obtain a vector normal to the collection of 3D points. The normal vector is projected from one point in the collection of contact points or from one point not in the collection but near the collection of points. The 3D coordinates of the contact point corresponding to the one point are obtained by projecting the normal vector from the 3D sphere center coordinates toward the surface being measured.
An error in the radius of the SMR will not change the overall shape of a planar surface. However, an error in the radius of an SMR can cause errors in other situations. This can readily be seen by considering the case in which an SMR is used to measure the distance between two planar surfaces, wherein one planar surface is to the left of the SMR and one planar surface is to the right of the SMR. If the true diameter is greater than the nominal diameter or reference diameter and if all other measurements are perfect, then the measured distance will be too small by twice the SMR radius error.
An error in SMR radius can also produce errors in measurements made on supports such as kinematic nests.
A way to describe the support axis 1430 in mathematical terms is as a locus of sphere centers for spheres of different sizes mounted on the nest. In other words, a sphere with a relatively small radius and a sphere with a relatively large radius will both lie on the support axis 1420. Of course, there is some minimum and maximum sphere size that will properly fit onto the three balls 1420, but within the acceptable range of sphere sizes, the locus of sphere centers will lie on the support axis 1430.
SMRs are provided by manufacturers with a nominal radius. Application software performs calculations based on the nominal radius of an SMR, the nominal radius being a numerical value provided by the SMR manufacturer. Differences in values for the nominal radius and the actual radius can result in errors, as is discussed further hereinbelow.
A nest, such as nest 14A, does not in general have a clearly defined reference point. A convenient and consistent reference point for a combination of a particular nest with a particular SMR is an “ideal support position,” which is a position on the support axis 1430. The ideal support position is defined as the position on the support axis 1430 of the sphere center for a sphere having a radius exactly equal to the nominal radius. An SMR having a radius different than the nominal radius will have a sphere center at a different position on the support axis referred to as the actual support position.
Because neither measurement of a radius nor corrections to 3D coordinates based on radius error depend on the internal reflecting mechanism of the retroreflector, the methods for correcting radius discussed herein are not limited to an SMR having an open-air cube corner retroreflector. The methods are equally applicable to an SMR that includes a glass cube corner retroreflector, which is to say, a retroreflector that includes a glass prism having three perpendicular reflecting faces. The methods are also applicable to the case of a cateye retroreflector in which the retroreflector has a glass optical element shaped either as a single sphere or as two hemispheres cemented together, wherein the optical element is placed at least partially within a cavity in a spherical exterior portion. SMRs that include glass prism or cateyes are well known in the art.
As stated hereinabove, the measured radius minus the nominal radius is the radius delta. Let the support axis 1430 be positive in a direction from a plane connecting the centers of the three spheres to the sphere center of an SMR held by the nest. In other words, in
The amount of shift in the sphere center 860 is now calculated for the case of an SMR 700 supported by a kinematic nest 1400 having three balls 1420 separated by 120 degrees.
In general, the change in height δh t depends on the construction (geometry) of the SMR nest 1400 and is different for different types of nests. In order to correct as accurately as possible for the effect of the radius error on the change in height, it is necessary to know both the amount of radius error and the type of nest being used with the SMR. To get an approximate correction for the effect of the radius error on the change in height, the radius error e may be used without considering the geometry of the nest. For the example given in the last paragraph, the relative error in the calculated height resulting from not considering the geometry of the nest 1400 was 4.8%.
An example is now given of the effect of SMR radius errors on measurements made with SMRs in nests.
In contrast,
It is clear from the discussion hereinabove that in many situations the direction of the support axes 1430 of two nests can have a large effect on the uncorrected distance measured between SMRs placed in the nests. There are several ways that a direction of a support axis 1430 can be obtained. First, the operator may indicate the direction of the support axis in application software. Second, a nest may be mounted directly on an object having surfaces known in a CAD model, from which the direction of the support axes 1430 may be determined. Third, measurements may be made to an inspection plan that includes the direction of the support axis 1430 of each nest used in the inspection. Fourth, the operator may measure features of the nest to determine the direction of the support axis 1430. For example, in the case of the nest 1400 of
In general, to correct for errors for measurements made with SMRs placed in nests, both the direction of the support axis 1430 and the magnitude of the radius delta is needed. A more accurate correction is possible if the geometry of the nest is also taken into account as discussed with reference to
A procedure described with reference to
In a first step of a method, two or more SMRs 700A, 700B are rotated in supports 1400A, 1400B to align the axis of symmetry 840 of each SMR to a 3D measurement instrument located at a first position having 3D coordinates (x1, y1, z1) within a frame of reference 1810. At this first position, the tracker is given reference number 10A. This alignment may be achieved, for example, using one of the alignment methods of
The mathematical method for doing this is easy to understand. Using the measured values for the retroreflector vertices for 700A, 700B, 700C, the tracker 10B is put into the frame of reference of the tracker at 10A. This is done using optimization methods in which the six degrees of freedom (for example, x, y, z, pitch, roll, yaw) of tracker 10B are adjusted until the 3D coordinates for 700A, 700B, and 700C measured by tracker 10A and 10B match as closely as possible. The usual optimization method is one in which the sum of the squared residual errors is minimized.
The transformation matrix needed to 3D coordinates measured by 10B into the frame of reference of 10A can be used to transform the SMR error vectors for 700A, 700B in 10A into the error vectors for 700A, 700B in 10B. In this way, the step of realigning the SMRs 700A, 700B prior to measurement by tracker 10B can be eliminated.
It should be understood that the method of establishing a transformation matrix by measuring the 3D coordinates of at least three retroreflector targets from at least two locations may be carried out even in the absence of SMRs. In other words, cube corner retroreflectors or other types of retroreflectors may be affixed to any sort of object. It should also be understood that the method described herein may be used to correct the 3D coordinates of a single SMR or multiple SMRs when viewed from two or more stations.
As explained hereinabove, a type of preferred orientation of an SMR is one in which the SMR reference ray 940 is aligned in a prescribed manner to a measurement line as a way of minimizing error in measuring a dimensional characteristic associated with the measurement line. A case is now considered in which the dimensional characteristic of interest is a length between two points.
In a measurement shown in
In
The error in the measurement of
To simplify the alignment of the SMR runout, it is convenient for the operator to know the direction of the maximum runout error vector component. This is easily accomplished if the reference point is aligned to the maximum runout error vector component, in other words, if the SMR runout reference angle is set to zero. Alternatively, it could be set to 180 degrees or another easily understood value such as +90 or −90 degrees. This angle is a preferred and predetermined angle in the sense that each SMR produced by a manufacturer has the reference point at the same position relative to the maximum runout error vector.
The method of aligning a reference mark can also be applied to minimize errors in dimensional measurements in addition to the measurement of length. An example of a dimensional measurement of a small displacement in the positions of the first SMR 1982, 1983 in the direction x′. The orientation of the reference mark 932 shown in
To summarize, it is mathematically convenient and customary in the art to select a gimbal point 22 as a stationary point within the device 10 and to refer 3D coordinates measured by the device 10 back to this point. This means that mathematical compensations are made to account for non-ideal aspects of real-world mechanical axes. One such non-ideal aspect is the axis offset 29. Another non-ideal aspect of the mechanical axes is axis non-squareness. In an ideal mechanical system, the azimuth and zenith axis are exactly perpendicular. In a real system, there is small deviation from perpendicularity, which is an angular value called the axis non-squareness value. To get the measured 3D coordinates, mathematical methods are used to correct the distance and two angles measured by the device 10 to refer these values to the gimbal point 22.
In a similar manner, in an ideal device 10, the beam of light 46 passes virtually through the gimbal point 22. In a real system, the beam of light is offset from the gimbal point 22 first by an axis offset distance 29. In addition, the beam of light 46 is offset from the beam rotation point (on the zenith axis 18). The offset is a small distance value in y″ and z″ in the y″-z″ plane of the payload frame of reference 35 shown in
In an ideal device 10, the beam of light 46 emerges from the device in a direction perpendicular to both the azimuth axis and the zenith axis. In a real device 10, the beam of light 46 emerges slightly off this idea angle. To move measured distance and two angles to coordinates with the tracker frame of reference, with the gimbal point as the origin, parameters may be used to mathematically account for these effects and reference all measurements to the origin. For example, in the type of beam steering mechanism described in device 10 of
It is necessary to provide a reference distance for the distance meter within the device 10. Since the beam of light from the distance meter appears to emerge from the beam rotation point, it is necessary to provide a way to refer any measured distances to this point. Methods for doing this are discussed in detail hereinbelow.
The frontsight mode of the device 10 is its usual mode of operation. The device illustrated in
Another mode of operation of the device is the backsight mode. To get into the backsight mode, starting from the frontsight mode, the payload is rotated about the azimuth axis by 180 degrees and rotated about the zenith axis to point the beam of light 46 back at the retroreflector target. As shown in
To establish the beam rotation point as the zero point for the distance meter, a separate procedure is carried out as is now described with reference to
The gimbal point 3112 of the device 3110 is placed at a point on the exterior portion. The distance 3124 of the beam of light 3114 from the gimbal point 3112 to the vertex point 3122 of the SMR in the first nest is measured. The SMR is moved to the second nest 3130′. The distance 3126 of the beam of light 3116 from the gimbal point to the vertex point 3122′ of the SMR in the second nest is measured. The distance 3126 minus the distance 3124 is the distance between the vertex point 3122 and the vertex point 3122. However, for the geometry of
Two additional steps illustrated in
The distance 3126 should equal the sum of distances 3142 and 3144 if the distance meter is correctly zeroed to the beam rotation point (on the zenith axis 18). To correct for any discrepancy from the ideal, an offset distance is calculated using the formula offset distance=(distance 3126−(distance 3142+distance 3144))/2. The offset distance is added to each distance reading. Equivalently, we can consider that the offset distance is set to zero the beam rotation point.
There are two different types of distance meters—absolute and incremental—that are reset somewhat differently. In the case of an incremental distance meter such as an interferometer, the distance is set to a known value at a given point. For example, it could be that a laser beam sent to an SMR sent to a home position nest is expected to have a distance reading of 0.167 meters based on measurements made in a factory or laboratory. For an incremental distance meter, a beam of light is sent to the SMR in the home position and set to read a value of 0.167 meter. Afterwards, the device may count the number of wavelengths shift in waves of light and multiply these by the wavelength of the light in question (in the local air medium) to get the total change in distance. This change in distance is added to the original distance to get the distance at any later time.
In the case of an absolute distance meter (ADM), a beam of light may again be sent to a distance meter, but in this case the distance read by the distance meter is compared to the known distance, which might again be 0.167 meter. One or more parameters associated with the measurement of the ADM, such as a phase offset parameter, for example, are adjusted to give the expected reading of 0.167 meter. Thereafter the adjusted parameter(s) continues to be used in making corrections to the ADM readings.
Although the way of resetting a distance meter is somewhat different in the two cases—resetting to a distance for the incremental distance meter and resetting parameters for the ADM—in both cases, it is necessary to send the beam of light from the device to the vertex of the SMR before making the necessary compensations.
The methods for setting the zero distance value for the distance meter in the device 10 discussed hereinabove have shown the distances measured by the distance meter taken with respect to the SMR vertex points 820. However, each SMR has its own depth error, which means that the method of
Some variations are possible in the procedures described in reference to
In another case, different SMRs may be placed in the nests 3130 and 3130′ rather than shifting one SMR between nests. Using two SMRs in this way may save time in an automated procedure. In this case, the depth error of each SMR is accounted for separately in determining setting the distance meter to zero at the beam rotation point.
The discussion with regard to
For a device that does not have a home position nest, the end result of the procedures of
As stated in the preceding paragraph, for a device having a nest that holds an SMR, the end result of the procedures of
In routine use of the device 10, the home reference distance is used to correct the distance reading of an SMR placed at the home position nest 17. Such routine correction may be useful in correcting drift in the distance meter, which may occur over time and as a result of temperature changes and mechanical shocks. As explained in
In some cases, it is not possible to align the axis of symmetry of the SMR 700 with the beam direction 46. For example, in
In this case, it is advisable to account for the effects of the SMR runout error vector component as well as the SMR depth error. This may be done in two steps. In a first step, the SMR is placed in the nest with the collar a certain distance off the nest surface. For example, a prescription might be to lower the collar until it touches the nest surface and then raise the nest by 2 mm. Assuming that the operator can adjust the collar to within one millimeter of the desired value for an SMR having a radius of 19.05 mm, alignment is obtained to within about ±3 degrees. In a second step, the SMR is aligned to place the reference point 932 at a specified orientation. For example, the prescription might be to reference point 932 at the uppermost SMR position. The calculations discussed hereinabove with respect to
An electrical memory component within 2000 may be used to store information. Such information may be used by the processor or may be transmitted to a remote processor by wired or wireless means. The information may include a serial number and SMR parameters as discussed hereinabove.
While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.
The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims
1. A probing system comprising a spherically mounted retroreflector (SMR), the SMR including a body, a retroreflector, and a temperature sensor, the body having a spherical exterior portion that has a sphere center, the body containing a cavity, the cavity sized to hold the retroreflector, the cavity open to a region outside the body, the retroreflector at least partially disposed in the cavity, the retroreflector having a set of three mutually perpendicular planar reflectors that intersect in a first set of three lines and in a common vertex point, there being a first distance between the vertex point and the sphere center, the temperature sensor embedded in the body of the SMR and in thermal contact with the SMR, the SMR further including a socket rigidly affixed to the body, the socket in electrical contact with the temperature sensor, the socket configured to accept a first electrical connector, the socket accessible to the first electrical connector from an outside surface of the SMR, the first electrical connector attached to a first end of an electrical cable.
2. The probing system of claim 1 further comprising a first electrical circuit, the first electrical circuit configured to be attached to a second end of the electrical cable, the first electrical circuit configured to provide electrical power to the temperature sensor, the temperature sensor being at a first temperature, the first electrical circuit configured to receive a first electrical signal from the temperature sensor and to provide a second electrical signal in response, the second electrical signal being responsive to the first temperature.
3. The probing system of claim 2 wherein the first electrical circuit is configured to provide the second electrical signal to a device, the device configured to measure three-dimensional (3D) coordinates of the vertex point and to determine 3D coordinates of the sphere center based at least in part on the second electrical signal and the 3D coordinates of the vertex point.
4. The probing system of claim 3 wherein the first electrical circuit is configured to provide the second electrical signal to the device as a wireless signal.
5. The probing system of claim 4 wherein the first electrical circuit includes an antenna, the antenna configured to transmit the wireless signal to the device.
6. The probing system of claim 5 wherein the first electrical circuit further includes a battery.
7. The probing system of claim 6 wherein the first electrical circuit includes an enclosure.
8. The probing system of claim 6 further including a glove configured to fit over a hand of an operator.
9. The probing system of claim 6 further including a strap or band configured to hold the probing system against a wrist of an operator.
10. The probing system of claim 3 wherein the SMR further includes an information storage element, the information storage element selected from the group consisting of a one-dimensional barcode, a two-dimensional barcode, and an radio-frequency identification tag, wherein the information storage element includes at least one temperature-related SMR parameter.
11. The probing system of claim 10 wherein the at least one temperature-related SMR parameter is a change in the first distance per unit change in the first temperature.
12. The probing system of claim 1, wherein:
- the socket is disposed on or in the body of the SMR.
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Type: Grant
Filed: Dec 11, 2013
Date of Patent: May 3, 2016
Patent Publication Number: 20140098382
Assignee: FARO TECHNOLOGIES, INC. (Lake Mary, FL)
Inventors: Robert E. Bridges (Kennett Square, PA), Kenneth Steffey (Longwood, FL)
Primary Examiner: Kara E Geisel
Assistant Examiner: Rufus Phillips
Application Number: 14/102,889
International Classification: G01B 11/14 (20060101); G01B 5/00 (20060101); G01B 11/00 (20060101); G01S 17/66 (20060101); G01S 7/00 (20060101); G01S 7/497 (20060101); G01C 15/00 (20060101); G01C 15/06 (20060101);
